Vitamin A is a known compound represented by the formula: ##STR1##
Carboxylic acid ester derivatives such as acetates or palmitates of vitamin A have widely been used as medicaments, food additives, feed additives and the like. These vitamin A derivatives have been produced by various methods.
These methods include the following two processes for producing vitamin A which involve the Wittig or its related reactions using organic phosphorus. ##STR2## wherein Ph is a phenyl group, and X is halogen (Pure Appl. Chem.43, 527 (1975 )). ##STR3##
The above processes for producing vitamin A have the following problems.
The above process No. 1, which uses the Wittig reaction, produces a large amount of cis-isomers such as 9-cis-vitamin A-acetate of the formula: ##STR4## and triphenylphosphine oxide as by-products, and produces the desired product in low yield. Separation of these by-products is accompanied by complex operations and is difficult. In particular, since triphenylphosphine oxide is insoluble in water, it is very difficult to separate it from the resulting vitamin A-acetate.
The above process No. 2, which uses the Wittig-Horner reaction, cannot produce carboxylic acid esters such as an acetate or palmitate of vitamin A directly.
One reference reports on a process to obtain, such carboxylic acid esters of vitamin A, wherein an alkyl ester of retinoic acid obtained by the above process No. 2 is reduced to vitamin A, followed by acylation to give a carboxylic acid ester of vitamin A (see Angew. Chem. 72, 811 (1960)). This process is not suitable as an industrial process because the reducing agent used to reduce the alkyl ester of retinoic acid is expensive and the process involves many complicated reaction steps.